Physics of Virus Transmission by Speaking Droplets

Droplets of oral fluid emitted by speaking are a long-recognized mechanism of respiratory virus transmission. While there have been many simulations of droplet evaporation to determine whether droplets containing virions remain floating in air or rapidly fall to the ground, they typically conceal the fundamental mechanisms because of the use of numerics. To make the physics of emitted oral fluid droplets easily understood, we present simple and transparent algebraic equations that capture the essential physics of the problem. Calculations with these equations provide a straightforward way to determine the airborne lifetime of emitted droplets after accounting for the decrease in droplet size from water evaporation. At a relative humidity of 50%, droplets with initial radii larger than about 50 microns rapidly fall to the ground while smaller, potentially-virus containing droplets shrink in size and remain airborne for many minutes. Rough estimates of airborne virion emission rates while speaking support the proposal that covering the mouth can help end the pandemic more quickly.